1. Technical Field
The present invention relates to a calibration technique (offset correction technique) of azimuth measurement data in finding an azimuth direction using a geomagnetic sensor. In particular, it relates to an azimuth data computing method, a compass sensor unit, and a portable electronic device, which can perform proper calibration even when measurement data obtained from a three-axis geomagnetic sensor are confined in a peculiar plane, whereby a correct azimuth direction is found.
2. Background Art
There is known a conventional portable terminal, such as a cellular phone set, which is provided with a magnetic sensor for detecting geomagnetism to find a direction based on the geomagnetism detected by the magnetic sensor. The determined azimuth direction is used, for example, to display a map in proper orientation. A new portable terminal has recently appeared on the market, which is provided with a GPS (Global Positioning System) receiver for detecting a position to display a map around its current position in such a manner to orient the map in accordance with the heading of the portable terminal relative to the azimuth direction.
However, since there are extraneous magnetic fields leaked from a loudspeaker, a microphone, a magnetized metallic package for electronic parts, etc. mounted in the portable terminal, a magnetic sensor mounted in the portable terminal detects a mixture of the geomagnetism and other magnetic fields generated from the electronic parts and the like inside the portable terminal. The extraneous magnetic field may cause varying offset in measurement of the azimuth direction. Therefore, calibration is necessary to correct magnetic errors (that is offset) due to the magnetic fields generated from the electronic parts and the like inside the portable terminal.
In the case of a portable terminal with a two-axis geomagnetic sensor mounted in it, calibration is performed in such a manner that a user rotates the portable terminal, for example, 180 degrees to collect measurement data from the magnetic sensor during the rotation of the portable terminal so as to estimate offset based on the measurement data.
A technique for calibrating the magnetic sensor mounted in such a portable terminal is disclosed, for example, in Japanese patent laid-open No. 2004-12416. In this technique, the portable terminal is rotated at predetermined angles to estimate offset based on data measured at each angle by the magnetic sensor, thereby performing calibration irrespective of the rotating speed.
However, even in the method disclosed in Japanese patent laid-open No. 2004-12416, the user has to perform calibration by consciously rotating the portable device with the magnetic sensor mounted in it. Although it is less complicated than other conventional methods, since it requires the user to follow the calibration procedure, it remains burdensome for the user to follow the procedure. In particular, in the case of a three-axis geomagnetic sensor, since the calibration requires three-axis data, the user is required to follow a more troublesome procedure.